1. Field of the Invention
The present invention relates to a method of manufacturing a flash memory device, and more specifically, to a method of manufacturing a flash memory device in which program, erase and read operational characteristics can be improved.
2. Discussion of Related Art
In a flash memory device, a dielectric film has an absolute influence upon program, erase and read characteristics in a memory cell transistor having a stack type gate structure.
Generally, a dielectric film has an ONO1/2/3 (HTO/nitride/HTO) structure. Oxide films corresponding to ONO1 and 3 are deposited by means of DCS based or MS based CVD (Chemical Vapor Deposition) method. The CVD oxide film formed by this chemical vapor deposition has a film quality lower than the oxide film typically formed through dry or wet oxidization. More particularly, the film quality and thickness of the ONO3 oxide film have an absolute influence upon charge leakage and retention properties of a memory cell.
Further, the nitride film corresponding to ONO2 has lots of apertures and is likely to be easily broken. Thus, it is required that pin holes, etc. existing in the nitride film be controlled by performing a high temperature annealing process as a subsequent process.
Accordingly, in order to obtain the charge leakage and retention properties required in the operation of a device in the flash memory device of the stack type structure, it is required that the film quality of ONO3 (HTO) as well as an ONO2 (nitride) thin film be improved through a high temperature wet annealing (for example, steam annealing) process after the ONO1/2/3 (HTO/nitride/HTO) thin film of a predetermined thickness is formed. However, as the wet annealing process of a high temperature is performed under wet atmosphere of a high temperature over a long time at the atmospheric pressure, thermal budget can be generated. Moreover, abnormal oxidization due to generation of punches of the ONO2 (nitride) thin film causes the dielectric constant of the dielectric film to change. Therefore, the program, erase and read operational characteristic of the flash memory cell can be degraded.